1. Field of the Invention
The invention is directed to a method for adding two or more power amplifiers in parallel, and balancing the current between the parallel joined amplifiers.
2. Prior Art
Paralleling of amplifiers (fast four quadrant DC to AC power converters) has been done for some time, but presently, the amplifiers are changing from linear to switch-mode technology. Also the environment in which they operate is continuing to demand larger amounts of power. When parallel amplifiers do not share current, costly inefficiencies arise.
At first paralleling of amplifiers was done by using simple passive ballasting. Linear amplifiers had wide bandwidth and fairly small phase errors which led to substantial conformity of gain and phase characteristics. High frequency circulating currents were reduced by using a highly coupled center tapped inductor whose center tap joined to the loads and whose ends attached to an amplifier output. If the amplifiers are delivering equal currents, such as inductor will store no net energy and thus no signal voltage will be lost to inductance. It is important not to loose signal voltage as the cost of generating large amounts of power are also large.
When the demands on the ballast resistors grew to more than 250 Watts of dissipation, negative current feedback was used to synthesize an effective amplifier output resistance (lossless). This constituted a second and improved generation of paralleling design.
With the advent of high efficiency switch-mode amplifiers additional issues have arisen. Output currents are typically larger and the gain and phase characteristics are now much looser in tolerance, potentially making current sharing more difficult.
One of the preferred uses of the subject paralleled amplifiers is in the medical industry, for use with magnetic resonance imaging (MRI), where the load on the system is the gradient coil of the MRI device. This environment is relatively hostile for gradient signal processing, because the MRI device has large amounts of peak RF power (&lt;=20 KW) supplied to coils which are immediately inside the gradient coils. With such intimate coupling, it is necessary to place low-pass filters in the feed lines to the gradient coils to contain the RF currents. These filters tend to aggravate an already bad situation for establishing wide bandwidth negative current feedback. Large phase response lags within the amplifiers and distributed capacitances in the gradient coils already have limited the amounts of feedback that can be used to control the system. Any controls added to effect current sharing dare not corrupt the output signal as there is insufficient feedback to correct any significant injected non-linear errors. Therefore some of the methods practiced by the DC to DC converter industry for current sharing are not applicable here.
What is desired is a lossless means of sensing circulating (unbalance) currents caused by mismatched parallel power converters and introducing output corrections in such a manner as to not influence the net output available to the load. This implies that the entire method is lossless and also has no net output inductance added to the load circuit.